1. Field of the Invention
The invention relates to water-redispersible dispersion powders that are stabilized by protective colloids and contain polyvinyl alcohol having a Höppler viscosity of not more than 3 mPas, their preparation and their use in hydraulically setting systems.
2. Background Art
Polymers based on vinyl esters, vinyl chloride, (meth)acrylate monomers, styrene, butadiene and ethylene are used first and foremost in the form of their aqueous dispersions or water-redispersible polymer powders in many applications. For example such polymers are used as coatings or adhesives for a wide variety of substrates. These polymers are stabilized using protective colloids or more rarely low molecular weight surface-active compounds. Polyvinyl alcohols are generally used as protective colloids.
Polyvinyl alcohol has many advantages as a protective colloid. In particular, polyvinyl alcohol makes powder stable against premature coalescence of the particles while acting as binder. However, polyvinyl alcohol is sometimes disadvantageous when large amounts of dispersion powder are used (e.g., in sealing slurries). In this case, the newtonian rheology of the polyvinyl alcohol is a disadvantage in the processing of these compositions.
The polyvinyl alcohols used according to the prior art generally have a Höppler viscosity (in accordance with DIN 53015: 1978-09) of at least 3 mPas. In other prior art variations, the polyvinyl alcohols have a Höppler viscosity of at least 4 mPas, as described, for example, in patent documents DE 10162513 A1 and EP 1 262 465 B1. Although there is no upward limit on the Höppler viscosity, values above 30 mPas are seldom employed due to the increasing viscosity.
Values of the Höppler viscosity below 3 mPas have not been studied very well because of the assumption that such low molecular weight compounds are not very suitable for stabilizing dispersion powders.
Dispersion powders are prepared, for example, by spray drying, as described in EP 1 262 465 B1, or by freeze drying of aqueous lattices.
The products set forth above are used, in particular, as binders in hydraulically setting adhesives. For example, these products are used in tile adhesives based on cements or gypsum plaster.